In the production of sulfuric acid and oleum, it is a common practice to subject a gas stream containing SO.sub.2 and oxygen to catalytic reaction on a succession of contact-catalyst hurdles, thereby producing SO.sub.3 which can be absorbed in water or sulfuric acid.
In the transformation of SO.sub.2 containing gases to SO.sub.3 in, for example, sulfuric acid manufacturing processes, it is known that the catalyst must be operated at a temperature above the so-called starting or threshold temperature for effective conversion. At temperatures below this critical starting temperature, the conversion does not proceed to any significant degree. In the case of vanadium pentoxide (V.sub.2 O.sub.5) catalysts, depending upon the composition and the method of production, the starting or threshold temperature is between 400.degree. and 450.degree. C.
Since the conversion of SO.sub.2 to SO.sub.3 is an exothermic reaction, the heat evolved during the reaction frequently raises the temperature of the catalyst bed. For example, with gases having an SO.sub.2 concentration up to about 11%, the temperature can rise to 620.degree. C, a temperature at which the equilibrium condition SO.sub.2 +1/20.sub.2 .revreaction.SO.sub.3 is achieved. With so high a concentration, the temperature may rise above the equilibrium temperature in which case the reverse reaction dominates, i.e. the equilibrium favors high SO.sub.2 and low SO.sub.3 concentrations in the equilibrium mixture. As a consequence, operations above the SO.sub.2 +1/20.sub.2 .revreaction.SO.sub.3 equilibrium are undesirable. Furthermore, at temperatures above about 620.degree. C, the catalyst is endangered. Catalysts which have an upper temperature limit at which they remain effective and which have a tendency to lose potency above a critical temperature level are referred to hereinafter as temperature-sensitive catalysts.
It has long been recognized in the art that it is important, because of the temperature sensitivity of the catalyst, to prevent overheating of the catalyst in SO.sub.2 conversion systems.
Thus it is known to lower the SO.sub.2 concentration of the gas by withdrawing from the first contact catalyst stage the gas which has been partially transformed to SO.sub.3 and to recirculate it to the incoming gas. Of course, where a portion of the gas is continuously recirculated and all of the incoming gas plus the recycled portion must be passed through the first contact stage, a large volume of gas must be processed, especially since the recirculated quantity of gas must also be large to bring about the desired reduction in SO.sub.2 concentrations. In fact, this latter volume must be increased with higher SO.sub.2 concentrations and as a result the processing equipment must be of corresponding volumetric capacity.
It has also been proposed to mix with the SO.sub.2 -containing incoming gases, SO.sub.3 -containing air as is derived from the formation of oleum (i.e. the blowing out thereof). This technique is only practiced where oleum is to be produced and also increases the volume of gases which must traverse the contact vessels.
Gases with an SO.sub.2 content up to 14% and with an oxygen deficiency can be treated in stages and completed in stages by blowing dry cold air into the gas to increase the oxygen content and to cool the gas. This process, where oxygen is admitted between successive stages, has the disadvantage that uniform mixing of the gases with air requires expensive apparatus and complex control means. For example, the vessel cross-section must increase proportionally to the gas volume from stage to stage.
In still another prior art technique, gases with an SO.sub.2 content of about 8 to 11% are passed through a main contact vessel at a velocity of 0.6 to 2 meters per second while a branch stream is subjected to precontact so that an SO.sub.3 -containing effluent is formed which is returned to the main gas stream before it enters the main contact vessel. The transformation of the branch stream in the precontact stage takes place well below the SO.sub.2 +1/2 O.sub.2 .revreaction.SO.sub.3 equilibrium even with relatively low SO.sub.2 levels. In fact, it has proved to be impossible to obtain an equilibrium condition with SO.sub.2 concentrations above 11% in a system using a precontact stage as described.
It should also be mentioned that studies have been made on controlling the temperature maximum for the transformation of SO.sub.2 containing gases in a hurdle contact system and techniques have been described which require a large number of contact hurdles with high pressure drops and power losses through the system.